1. Field of the Invention
The present invention relates to an automatic exposure control device for a camera, and more particularly to an improved automatic exposure control device which can control exposure more precisely than a prior art device of similar type.
2. Related Background Art
Most of the recently manufactured cameras are equipped with automatic exposure control devices (referred to as AE hereinafter) so that light intensity measurement, diaphragm control and shutter speed control are automatically carried out. However, the shutter speed which has the biggest influence to the determination of exposure is subject to variation by various reasons because the shutter is a mechanical apparatus. As a result, even in a camera with AE, the exposure may deviate from a proper value.
Such conditions which affect the shutter speed are (i) differences in shutter charaacteristics of individual cameras due to tolerance of parts of the respective shutters (ii) affects of temperature, humidity and gravity in the environment of use of the camera, and (iii) aging or abrasion of the parts due to frequent use of the camera.
In order to attain proper exposure even when there is a variation in the shutter speed by any of the above conditions, it is necessary to detect an actual shutter speed and correct the exposure based on the detected shutter speed. However, AE on an external light intensity measurement camera which has a light intensity measuring device independent from the shutter operation has no such function.
A type of shutter in which the variation of the shutter speed has a significant affect on the exposure is a lens shutter which shares a diaphragm and a shutter. In a camera with a lens shutter, a difference in the shutter characteristics of individual cameras, that is, a difference in the shutter speed, apts to create a variation in the proper exposure of the camera. Since the change of shutter aperture in the lens shutter is usually complex, the exposure time is a complex variable and a high precision exposure control is necessary to determine exact exposure. Accordingly, in the lens shutter, it is necessary to provide a stabilizing mechanism such as a governor in order to simplify the aperture change. However, this leads to an increase in the size of the camera and complexity of the shutter mechanism.